🎓 Lesson 3
D2
Calculating Net Acid Generation (NAG) & ANC
Net Acid Generation (NAG) tells us how much acid a rock sample will produce when exposed to air and water, while Acid Neutralizing Capacity (ANC) measures how much acid the same rock can neutralize — their difference shows whether the rock will make acidic or neutral drainage.
🎯 Learning Objectives
- ✓ Calculate Net Acid Generation (NAG) and Acid Neutralizing Capacity (ANC) from laboratory assay data
- ✓ Interpret NAG–ANC results to classify waste material using the Canadian Environmental Protection Act (CEPA) ARD prediction framework
- ✓ Explain the geochemical significance of the NAG pH test versus traditional total sulfur assays
- ✓ Apply correction factors for non-pyritic sulfides and reactive carbonates in ANC estimation
- ✓ Design a preliminary waste rock management strategy based on NAG–ANC classification
📖 Why This Matters
Over 70% of mine closure liabilities stem from acid rock drainage (ARD), which contaminates surface and groundwater with metals and sulfate for centuries. In 2023, the International Council on Mining & Metals reported that ARD-related remediation costs averaged $42M per legacy site in North America. Accurately predicting whether waste rock or tailings will generate acid is not just academic — it dictates disposal method, liner requirements, monitoring scope, and long-term liability. NAG and ANC are the cornerstone geochemical tools used globally to make these decisions *before* mining begins.
📘 Core Principles
Traditional total sulfur assays overestimate acid potential because they assume all sulfides oxidize completely — but real-world weathering is kinetically limited and mineral-specific. The NAG test (ASTM D7169/D7170) uses controlled peroxide digestion to selectively oxidize *reactive* sulfides (mainly pyrite and marcasite), simulating decades of natural oxidation in hours. ANC, measured via acid titration (e.g., ASTM D7573), quantifies carbonate buffering capacity — but must be corrected for non-carbonate alkalinity (e.g., from Mg-silicates) and acid-consuming clays. Critically, NAG and ANC must be expressed in *equivalent acid units* (kg H₂SO₄/t) for direct comparison — requiring stoichiometric conversions (e.g., 1 wt% pyrite ≈ 0.89 kg H₂SO₄/t; 1 wt% CaCO₃ ≈ 0.36 kg H₂SO₄/t). The resulting NAG–ANC value classifies material as potentially acid generating (PAG), non-acid generating (NAG), or transitional — guiding regulatory compliance and engineering design.
📐 Key Calculation
The core calculation compares acid generation potential (NAG) against neutralization capacity (ANC), both converted to equivalent sulfuric acid (H₂SO₄) mass per tonne of material. The NAG value is derived experimentally, while ANC is calculated from carbonate content and adjusted for reactive non-carbonate minerals. The net result determines ARD risk classification.
💡 Worked Example
Problem: A waste rock sample yields: Total S = 1.8 wt%, Reactive S (NAG-S) = 1.2 wt%, CaCO₃ = 3.4 wt%, MgCO₃ = 0.9 wt%. Calculate NAG, ANC, and classify using CEPA criteria (PAG if NAG–ANC > 20 kg H₂SO₄/t).
1.
Step 1: Convert NAG-S to NAG: 1.2 wt% × 0.89 kg H₂SO₄/kg S = 1.068 kg H₂SO₄/t
2.
Step 2: Convert CaCO₃ to ANC: 3.4 wt% × 0.36 kg H₂SO₄/kg CaCO₃ = 1.224 kg H₂SO₄/t
3.
Step 3: Convert MgCO₃ to ANC: 0.9 wt% × 0.41 kg H₂SO₄/kg MgCO₃ = 0.369 kg H₂SO₄/t (using MgCO₃ → H₂SO₄ stoichiometry)
4.
Step 4: Sum ANC = 1.224 + 0.369 = 1.593 kg H₂SO₄/t
5.
Step 5: Compute NAG–ANC = 1.068 – 1.593 = –0.525 kg H₂SO₄/t
Answer:
The result is –0.53 kg H₂SO₄/t, which is < 20 kg H₂SO₄/t and negative — classifying the material as Non-Acid Generating (NAG) per CEPA guidelines.
🏗️ Real-World Application
At the Red Chris Mine (BC, Canada), pre-feasibility NAG–ANC testing revealed that 68% of waste rock was PAG (NAG–ANC = +42 kg H₂SO₄/t), prompting redesign of the waste dump: segregation of PAG material into lined, water-controlled cells with lime addition, while NAG material was placed in unlined, dry cover zones. This avoided $120M in post-closure treatment costs and secured permits under BC’s Mines Act Regulation 195/2005. Crucially, initial total-S screening suggested only 41% PAG — underscoring why NAG–ANC, not total sulfur, is the regulatory benchmark.
🔧 Interactive Calculator
🔧 Open Mine Water Treatment & Resource Recovery Calculator📋 Case Connection
📋 Rare Earth Element Recovery from Phosphate Mine Wastewater – Florida, USA
REE concentrations low (1–5 ppm), but massive flow; competing Ca/P/SO₄ fouling ion exchange resins